Scientists have discovered traces of another world in rock samples from the Moon—and it's evidence of the planet that once crashed into Earth to form our rocky satellite.

It's been believed since the 80s that the Moon was created when a planet called Theia hit Earth 4.5 billion years ago. But until now, there hasn't been any evidence in lunar samples to confirm the theory.

Now, though, a new analysis of rock brought back to Earth by Apollo astronauts reveals subtle differences in the composition of the material which makes up the Moon compared to that of our planet. The researchers measured the difference in the isotopic composition of oxygen contained in rocks from Earth and Moon—that's the ratio of oxygen atoms with subtly different atomic weights.

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They found a difference between the rock samples—which, the researchers claim, confirms the theory that the Moon was created by a dramatic collision between Theia and Earth. Their results are published in Science.

However, the differences is smaller than the researchers expected. Similar analysis of meteorites from Mars and the outer solar system yield far larger disparities, so scientists are now scratching their heads over why this new analysis provides results which are so similar.

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Some have suggested Theia actually formed very close to Earth in the early days of our Solar System; others reckon that the impacting planet could have been bigger than we thought, which could have caused the Earth to pick up many of the qualities of its incoming rock, too. Regardless, we've now found evidence that Theia did indeed hit the Earth to form the Moon. [Science via BBC]